One of the greatest challenges in gas turbine design is the reduction of pollutant emissions. Among the most troublesome emissions from gas turbines are the oxides of nitrogen (NOx), which are known to have deleterious effects on the Earth's ozone layer. NOx emissions are also known contributors to acid rain and photochemical smog.
Gas turbines are being used in an increasing variety of applications, in part because of their high power-to-weight ratio, and their high energy efficiency. Thus, the need to reduce NOx emissions is becoming particularly acute. Emissions from ground-based gas turbines contribute to photochemical smog; emissions from conventional commercial and military aircraft contribute to the formation of acid rain. Most notably, the next generation of high-speed civil transport (HSCT) aircraft will fly in the stratosphere, directly affecting the Earth's ozone layer.
One of the prominent mechanisms for the creation of NOx in combustion reactions is known as "thermal NOx." In this process, the high thermal energy of the combustion reaction breaks the bonds of N.sub.2 molecules, forming NO and free N atoms. The free N atoms then combine with oxygen to produce further NO. In 1946, Zeldovich proposed a kinetic mechanism for the formation of thermal NO. The reaction can be described as follows: EQU N.sub.2 +O NO+N EQU N+O.sub.2 NO+O
The rate of NO formation in the Zeldovich mechanism is as follows: ##EQU1## where k.sub.f is the forward rate constant of the reaction EQU N.sub.2 +O.fwdarw.NO+N; ##EQU2## is the Boltzman Factor; R is the universal gas constant, 1.987 cal/K.gm.mol, T is the temperature, and E is the activation energy, measured in cal/mol. The forward rate constant has been empirically identified as 7.multidot.10.sup.13 and the activation energy is 75,000 cal/mol; therefore, thermal NO formation is extremely temperature dependent. Above a critical temperature of approximately 1900K, the production of NO dramatically increases. Therefore, decreasing the temperature of the combustion reaction is an effective method of reducing NOx emissions.
Notably, the next generation of high-speed civil transport (HSCT) aircraft, currently under development, are expected to operate with combustion reactions in excess of this critical temperature. However, because NO production increases exponentially with increasing temperature, reducing the variance of combustion temperature is also an effective method for reducing NOx emissions. It can readily be seen from the foregoing discussion that a small upward fluctuation in combustion temperature can dramatically increase NO production, but that a corresponding downward fluctuation will only slightly decrease NO production.
The reaction temperature in combustion systems is a function of the relative amounts of fuel and oxidant (typically air) used. The fuel-to-air ratio is said to be stoichiometric when the mixture, in theory, gives complete combustion, without any excess oxygen. It is well known in the art that when the fuel-to-air mixture is nearly stoichiometric, the reaction temperature is at its greatest. Therefore, some combustor designs burn "lean" or "rich"--that is, they utilize a fuel-to-air ratio that is either substantially less than, or substantially greater than, stoichiometric--in order to reduce NOx emissions.
Because of the exponential relationship between reaction temperature and NOx emissions, it is also understood in the art that complete mixing of the fuel and air prior to reaction is desirable. Proper mixing of the fuel and air prevents the formation of "pockets" having different fuel-to-air ratios. These pockets cause fluctuations in the overall reaction temperature, resulting in an increase in NOx emissions.
Conventional gas turbines typically inject fuel and air separately into the reaction zone. As a result, the fuel and air are not completely mixed prior to reaction. It is known in the art to provide a lean burn, direct-injected ("LDI") gas turbine combustor in which fuel and air are rapidly mixed prior to reaction at a lean mixture ratio.